1. Field of the Invention
The present invention relates to a connecting cable, a communication device and a communication method, particularly relates to a connecting cable in which an independent closed magnetic circuit interlinked with each signal conductor and formed by material provided with high magnetic permeability and predetermined magnetic reluctance is arranged for inhibiting crosstalk between signal conductors caused by the in-phase component of signals on two signal conductors, a communication device and a communication method.
2. Description of the Related Art
Recently, a device utilizing an interface according to the IEEE-1394-1995 high performance serial bus standard (an IEEE-1394 bus) for an interface for connecting plural information processors such as a computer and a video terminal is proposed.
FIG. 23 shows an example of an information processing system constituted by plural information processors which are respectively connected utilizing an interface according to the IEEE-1394-1995 standard (hereinafter, "IEEE-1394").
The above information processing system is constituted by a workstation 101, a personal computer 102, a hard disk 103, a printer 104, a scanner 105, an electronic camera 106 and a compact disc (CD)-ROM drive 107 respectively provided with an interface according to the IEEE-1394 standard.
The workstation 101 to the scanner 105 are respectively connected in a daisy chain mode via 1394 cables 111-1 to 111-4 according to the IEEE-1394 standard, and the electronic camera 106 and the CD-ROM drive 107 are respectively connected to the workstation 101 in a tree structure mode via 1394 cables 111-5 and 111-6.
FIG. 24 shows an example in which predetermined two devices 141A and 141B of the above workstation 101 to the CD-ROM drive 107 are connected.
The 1394 cable 111 is a cable according to the IEEE-1394 standard provided with two pairs of twisted pair signal conductors 12 and 13 (further provided with two power lines not shown in the case of a 6-pin cable) and provided with a 4- or 6-pin plug 125-1 or 125-2 at each end.
FIG. 25 shows an example (in the case of a 6-pin cable) of the section of the 1394 cable 111. As shown in FIG. 25, signal conductor shields 17-1 and 17-2 are respectively provided to each signal conductor 12 or 13 in the 1394 cable 111 and a whole cable shield 18 is provided outside the signal conductors 12 and 13 and the power lines 11-1 and 11-2.
The devices 141A and 141B shown in FIG. 24 are respectively provided with twisted pair A (TPA) interfaces 151A and 151B and twisted pair B (TPB) interfaces 152A and 152B respectively which are a part of an IEEE-1394 interface.
The TPA interfaces 151A and 151B and the TPB interfaces 152A and 152B respectively send/receive a signal between the two devices 141A and 141B and also respectively send/receive the arbitration information of cables determined in the IEEE-1394 standard and supplied from a predetermined device.
Further, the TPB interfaces 152A and 152B respectively supply a d.c. signal of voltage corresponding to any of plural types of maximum transfer rates determined in the IEEE-1394 standard to the TPA interfaces 151B and 151A of each connected device.
FIG. 26 shows an example of the electric constitution of each TPA interface 151A and 151B.
After a driver 161 amplifies a strobe pulse (Strb_Tx) corresponding to transmitted data when a strobe enabling signal (Strb_Enable) is supplied, the driver sends the amplified strobe pulse as a TPA signal via one of the two conductors of the signal conductor 12 or 13 and sends a signal generated by inverting a TPA signal as a TPA* signal via the other conductor of the same signal conductor.
For example, the driver 161 of the TPA interface 151A in the device 141A shown in FIG. 24 sends a TPA signal and a TPA* signal via the signal conductor 12.
An interface according to the IEEE-1394 standard adopts a DS linking system for encoding in data transmission. In the DS linking system, as shown in FIG. 27, predetermined data is transmitted on one signal conductor and a strobe pulse generated to change the value of the data when it is unchanged is transmitted on the other signal conductor. A clock pulse can be obtained by calculating the exclusive-OR of data and a strobe pulse.
A receiver 162 operates difference between signals transmitted via the two conductors of the signal conductor 12 or 13 and after the receiver amplifies the operated result, it outputs the amplified operated result as received data.
Arbitration comparators 163-1 and 163-2 respectively operate difference between signals corresponding to arbitration information and transmitted via the two conductors of the signal conductor as data, respectively judge whether a value showing the operated result is larger than a predetermined threshold value or not and respectively output a value corresponding to the judgement as received arbitration information.
A buffer 164 supplies predetermined reference voltage TpBias to a comparator 165.
The comparator 165 is provided with plural comparing sections not shown, compares the voltage value of a d.c. signal corresponding to the maximum transfer rate transmitted in a common mode (a mode in which a TPA signal and a TPA* signal are in phase) via the signal conductor 12 or 13 and preset reference voltage corresponding to plural maximum transfer rates (for example, 400 Mbps, 200 Mbps and 100 Mbps), and outputs the result of the comparison (the information of the maximum transfer rate of the connected device).
FIG. 28 shows an example of the electric constitution of the TPB interfaces 152A and 152B.
After a driver 171 amplifies a data signal (Data_Tx) to be transmitted when a data enabling signal (Data_Enable) is supplied, the driver sends the amplified data signal as a TPB signal via one of the two conductors the signal conductor 12 or 13 and also sends a signal generated by inverting a TPB signal as a TPB* signal via the other conductor of the same signal conductor.
A receiver 172 operates difference between signals transmitted via the two conductors of the signal conductor 12 or 13 and after the receiver amplifies the operated result, it outputs the amplified operated result as a received strobe pulse.
Arbitration comparators 174-1 and 174-2 respectively operate difference between signals corresponding to arbitration information and transmitted via the two conductors of the signal conductor 12 or 13 as data, respectively judge whether a value of the operated result is larger than a predetermined threshold value or not and respectively output a value corresponding to the judgement as received arbitration information.
A cable connection comparator 175 detects a voltage value varied because the cable 111 is connected and outputs the detected result.
When a signal (Speed_Tx) corresponding to the maximum transfer rate of a device in which constant current circuits 173-1 and 173-2 are built is supplied, the constant current circuits output current corresponding to the signal, generate predetermined voltage which is in phase (in a common mode) as a TPB signal and a TPB* signal and execute speed signaling processing.
Next, communication between the devices 141A and 141B shown in FIG. 24 will be described.
In the devices 141A and 141B connected via an interface according to the IEEE-1394 standard, first when a path is reset, the respective connected devices are informed in a common mode about the maximum transfer rate of the respective devices as speed signaling processing.
At this time, the TPB interfaces 152A and 152B of each device similarly apply voltage corresponding the maximum transfer rate of each device to the signal conductors 12 and 13 respectively in the constant current circuits 173-1 and 173-2 and when the TPA interfaces 151B and 151A of the devices connected to the above each device detect respective voltage values in the comparator 165, the devices connected to the above each device are informed about the maximum transfer rate of each device.
After each device is informed about the maximum transfer rate as described above, it starts the sending of data at the slowest transfer rate of preset plural transfer rates.
When data is sent, the driver 171 of the TPB interfaces 152A and 152B of each device sends data via one signal conductor and the driver 161 of the TPA interfaces 151A and 151B sends a strobe pulse corresponding to the data via the other signal conductor. The receiver 162 of the TPA interfaces 151B and 151A of the devices connected to each device receives a transmitted data signal and the receiver 172 of the TPB interfaces 152B and 152A receives a transmitted strobe pulse.
As described above, predetermined data and a strobe pulse corresponding to it are transmitted from one device to the other device according to the DS linking system.
However, there is a problem that as a magnetic flux interlinked with another signal conductor is increased of magnetic fluxes generated due to a transmitted signal in case a signal is transmitted in a common mode as in the above speed signaling processing, crosstalk between signal conductors is increased and a malfunction may occur in each device.
For example, a signal in a common mode sent from the TPB interface 152A in the device 141A shown in FIG. 24 via the signal conductor 13 is transmitted to the signal conductor 12 by electromagnetic induction, reaches the TPA interface 151A in the device 141A and the TPB interface 152B in the device 141B via the signal conductor 12 and crosstalk is caused.